smtpd.php: fixing GM_ALLOWED_HOSTS's first domain not allowed

current proposed is_host_allowed() implementation returns 0 when the current sender's email address domain is first in GM_ALLOWED_HOSTS's comma separated values. This makes if(is_host_allowed()) tests in other places in this file to fail because 0 is interpreted as false. Adding is_numeric() before return in is_host_allowed() to force sending either true or false, but never 0 (or another integer).